This Idea is sort-of a combination of a single bed with a
double-bed. It is known that the former is good for
individuals, and in fact so is the latter. But what if they were
combined?

If two people mostly want to sleep separately, a trapezoidal
mattress should do the job. Only their feet
would be
intertwined, and that is socially quite acceptable. Of course,
if the two people want to sleep together, this mattress would
allow that, too.